Polyol Ester Based Drive-Train Fluid for High Stress Automatic Transmissions

ABSTRACT

The use of certain polyol ester (POE) based fluids has been found to be surprisingly effective in, e.g., automotive automatic transmissions, such as, for example, racing automotive automatic transmissions. The use of select POE based fluids has been found to lower operating temperatures, improve torque convertor engagement and torque convertor lock-up due to improved efficiency, performance and friction properties, allowing improved efficiency in high stress applications.

This application claims benefit under 35 USC 119(e) of U.S. provisional application No. 61/764,748, filed Feb. 14, 2013, the disclosure of which is incorporated by reference.

This invention provides a thermally stable, high performance working fluid and a mechanical device comprising a fluid coupling and the working fluid, suitable for use in high stress systems such as automotive automatic transmissions operating under conditions of high horse power and high torque, an automatic transmission comprising a fluid coupling and said working fluid, and a method for improved functioning of a high stress power transfer system, e.g., an automotive automatic transmission, by adding the working fluid of the invention to the automotive automatic transmission prior to operation.

Transmission fluids, especially automatic transmission fluids, are required to serve a complex mixture of functions, including lubrication, corrosion prevention, heat dissipation, and hydraulic transfer fluid. For example, automatic transmissions contain a number of mechanical parts operating at close tolerances and require a fluid to lubricate these close-fitting parts to reduce friction and wear and keep down temperatures. Further, many automatic transmissions are hydraulically operated, that is they use a fluid coupling, such as a fluid flywheel or torque converter, to replace a manually operated clutch, requiring a hydraulic fluid to transfer energy from the engine, through the coupling, to the final drive mechanism.

In addition to serving as lubricant and hydraulic fluid, the automatic transmission fluid (ATF) needs to protect and preserve seals, resist foaming, transfer heat, prevent slippage, lower heat, maintain correct viscosity at a range of temperatures, be stable to high shear and thermal oxidative conditions and be compatible with standard additive packages used in the field, while preserving smooth shift quality.

Not surprisingly, automatic transmissions are used in a variety of applications where they are subjected to different degrees of stress. While many people are familiar with automatic transmissions in passenger cars, automatic transmissions are also used in, e.g., racing vehicles, buses, on/off highway trucks, marine propulsion systems, industrial conveyor drives, forklifts, winches, drilling rigs, construction equipment, and railway locomotives. Many of these uses create extremely stressful operational conditions for the transmission and the fluid.

For example, delivery trucks, such as postal delivery vehicles, subject transmissions to constant stopping and starting of heavily loaded vehicles and a preponderance of low gear use and shifting per mile driven. This creates an environment that may lead to failure of the transmission due to high frictional heat and wear, and breakdown of the fluid affecting both lubricating viscosity and hydraulic transfer properties while increasing foaming.

Another performance challenge for an ATF can be illustrated by the demands of auto and motorcycle racing, for example, drag racing, where automatic transmissions using torque converters produce high stall speeds to improve off-the-line torque and to get into the power band of the engine more quickly. That is, the car is stopped with the engine at a high rpm (the “stall speed”) to allow for a very quick launch when the brakes are released. This is even more advantageous for turbocharged engines, where the turbocharger needs to be kept spinning at high rpm by a large flow of exhaust in order to keep the boost pressure up and eliminate the turbo lag that occurs when the engine is idling and the throttle is suddenly opened.

In addition, continued improvements in technology are placing automatic transmissions in situations involving higher horsepower, higher torque and greater temperature ranges requiring fluids which meet continually more stringent physical tolerances.

Lubricating oils, naturally occurring and synthetic, are often used as base oils for ATFs. Poly-alpha-olefins are commonly employed as the base fluid, often in combination with other lubricants. For example, US 2010/0035778, incorporated herein by reference, discloses a power transmission fluid comprising a blend of at least two PAOs and a viscosity improver, in particular, a polymethacrylate.

U.S. Pat. No. 6,713,439, incorporated herein by reference, discloses an automatic transmission fluid composition comprising (a) from 1 to 49 wt. % of a polyalphaolefin base stock having a kinematic viscosity at 100 C of from 40 mm²/s to 500 mm²/s; (b) from 1 to 95 wt. % of a lubricant base stock having a kinematic viscosity at 100 C of from 2 mm²/s to 10²/s; (c) from 1 to 49 wt. % of a polyol ester of a C₅₋₃₀ aliphatic monocarboxylic acid and a polyol, and (d) an effective amount of a performance additive package selected from the group consisting of automotive gear oil additive packages, manual transmission fluid additive packages and automatic transmission fluid additive packages, provided that the composition has a kinematic viscosity at 100 C of at least 4 mm². The preferred polyol esters are esters of trimethylol propane. The ATF additive package of U.S. Pat. No. 6,713,439 comprises from about 1 to about 20% of the weight of the finished fluid and typically contains ashless dispersants; anti-wear agents; anti-oxidants; corrosion inhibitors; friction modifiers; seal swell agents; anti-foamants and sometimes viscosity modifiers, detergents, and pour point depressants

US 2005/0277557, incorporated herein by reference, discloses a thermally stable, friction, wear and degradation reducing transmission fluid for use in highly stressed power transmission systems comprising C₈/C₁₀ esters of a specific trimethyl polyol, a unique amide containing friction modifier, a viscosity index (VI) improver, a corrosion inhibitor, an antioxidant, dispersants, a lubricating oil flow improver, anti-foam agents and anti-wear additives.

U.S. Pat. No. 7,381,691, U.S. Pat. No. 5,972,854 and U.S. Pat. No. 6,726,855, all incorporated herein by reference, disclose various additives that can be incorporated into various lubricant base stocks which may be used in automatic transmission fluids.

U.S. Pat. No. 6,436,881 and U.S. Pat. No. 4,826,633, each of which is incorporated herein by reference, disclose lubricating base stocks containing mixtures of polyol esters, including polyol esters of pentaerythritol and di-pentaerythritol and C₅₋₁₀ carboxylic acids.

Often, attempts to improve one characteristic of an ATF will compromise another performance feature. For example, when formulating automatic transmission fluids for fuel economy, a common approach is to lower the viscosity of the fluid to reduce viscous drag. However, this approach can introduce number of potential negative side effects such as reduced lubricating film thickness, increased leakage losses, higher oxidation due to the lighter base stocks, etc.

It has been found that certain select mixtures of pentaerythritol and di-pentaerythritol esters similar to those U.S. Pat. No. 6,436,881 and U.S. Pat. No. 4,826,633, when used as the predominate component of a working fluid, e.g., the base oil of an automatic transmission fluid, surprisingly provide a hydraulic fluid/lubricant with excellent properties ideally suited for devices operated in high stress environments that employ a fluid coupling, such as automatic transmissions used in cars, busses, trucks, racing vehicles, industrial and construction equipment and the like.

SUMMARY OF THE INVENTION

The present invention provides a high performance working fluid for automatic transmissions or other mechanical energy transfer systems that employ a fluid coupling such as a fluid flywheel or torque converter, said working fluid comprising from 80 to 99% by weight, based on the total weight of the fluid, of certain neopentyl esters, in particular, esters formed from carboxylic acids and pentaerythritol, dipentaerythritol, tripentaerythritol and/or higher pentaerythritol oligomers, and from 1 to 20% additives such as ashless dispersants; anti-wear agents; anti-oxidants; corrosion inhibitors; friction modifiers; seal swell agents; anti-foamants viscosity modifiers, detergents, pour point depressants and the like. The fluid of the invention is well suited for use in automatic transmissions used in demanding environments such as those involving high horse power, high torque, heavy loads, high heat and wide temperature ranges etc.

For example, one embodiment of the invention provides a mechanical device comprising a fluid coupling and the working fluid of the invention. For example the device can be a transmission or power transfer system comprising a fluid coupling as found, e.g., in racing vehicles, buses, on/off road trucks, marine propulsion systems, industrial conveyor drives, forklifts, winches, drilling rigs, construction equipment, railway locomotives etc., as well as passenger cars. Racing vehicles include any vehicle involved in racing, for example, dragsters, stock cars, motorcycles, trucks, monster trucks etc. and vehicles used in off road racing, e.g., trucks, SUVs, all-terrain vehicles etc. On/off road trucks include e.g., delivery trucks, pickup trucks, etc.

The working fluid of the invention typically has a kinematic viscosity at 40° C. of from 4 to 64 cSt, for example, 5 to 48 cSt or 10 to 40 cSt, and in some embodiments 12 to 32 cSt; a kinematic viscosity at 100° C. of from 2 to 28 cSt, for example, from 2 to 10 cSt, 2 to 8 cSt, 2 to 6 cSt and in some embodiments 4 to 8 cSt; and a viscosity index of greater than greater than 100, generally at least 110 or 120, and in many embodiments at least 130 or 140. The fluid is stable at high temperatures and fluid at low temperatures with a pour point of −25° C. or lower, in many embodiments −30° C. or lower, −40° C. or lower, in some embodiments −50° C. or lower.

The working fluids of the invention maintain excellent lubricity and hydraulic energy transfer characteristics under heavy load, high temperature, high torque and high rpm and are compatible with standard additives common in the field. The fluid also has excellent anti wear activity, anti-swell activity, low volatility, low foaming, excellent resistance to oxidation, excellent heat transfer properties and friction characteristics that allow for lubrication at metal surfaces where needed, but which do not interfere with the desired slip/anti-slip features related to proper engagement of the various components of, e.g., a torque converter.

While the working fluids of the invention possess characteristics associated with high performance lubricants, e.g., anti-wear activity, anti-swell activity, low volatility, lubricity etc., in certain aspects, existing high performance automatic transmission fluids, using petroleum based oils, synthetic poly-alpha olefins and synthetic esters as base oils, actually exhibit a higher degree of activity in standard tests used in the evaluation of commercially viable lubricants. For example, commercial samples of high performance lubricants used in automatic transmissions found in racing cars and trucks, e.g., monster trucks and drag racing cars, performed at a higher level than a fluid according to the instant invention in Cameron Plint friction reduction and anti-wear test, and in ASTM D-4172 4-ball wear testing.

Despite the results from these individual tests, the fluid of the invention surprisingly delivered superior performance, e.g., lower temperatures and improved torque converter performance, compared to the commercial products when tested in the automatic transmission itself.

DESCRIPTION OF THE INVENTION

One embodiment of the invention is a mechanical system or device, comprising a fluid coupling and a working fluid, e.g., a hydraulic transfer fluid, said working fluid comprising:

-   -   i) from 80 to 99% by weight of a polyol ester composition         comprising predominately C₄₋₁₀ alkylcarboxlate esters of         pentaerythritol, di-pentaerythritol, tri-pentaerythritol and/or         higher pentaerythritol oligomers, for example, alkylcarboxlate         esters of pentaerythritol and di-pentaerythritol; and     -   ii) 1 to 20% by weight of additives; wherein the working fluid         has a kinematic viscosity at 40° C. of from 4 to 64 cSt, a         kinematic viscosity at 100° C. of from 2 to 28 cSt, and a         viscosity index of greater than greater than 100.

Typically, the additives of (ii) are selected from ashless dispersants; anti-wear agents; anti-oxidants; corrosion inhibitors; friction modifiers; seal swell agents; anti-foamants viscosity modifiers, detergents, pour point depressants and the like, but in some cases, small amounts, e.g. less than 15 wt %, 10 wt % or 5 wt % of other polyol esters or other lubricating oil may be included as an additive. Most often, other lubricating oils, e.g., trimethylol esters or poly-alpha-olefins are absent, or present at levels below 2 wt %.

The additives are well known in the art, many of which are found in the references already incorporated by reference, for example, antioxidants can include common amine, phenolic and phosphorus anti-oxidants, corrosion inhibitors can include triazoles, carboxylic acids etc., anti-wear agents, viscosity improver/pour point depressant can include various esters, phosphonic acids amines, amides methacrylates, mercaptans, alkanes, and pyrrolidone derivatives, and so on. Ionic liquids have found recent use in some lubricant compositions but are not a component in the present working fluid.

For example, the high performance working fluid comprises:

-   -   i) from 80 to 99% by weight of a polyol ester composition         comprising, or consisting essentially of, C₅₋₁₀alkylcarboxlate         esters of pentaerythritol and di-pentaerythritol; and     -   ii) 1 to 20% by weight of additives typically selected from         antioxidants, corrosion inhibitors, anti-wear agents,         dispersants, VI improvers, and pour point depressants.

In many embodiments the fluid coupling of the invention is a fluid flywheel or torque converter as found, for example, in an automatic transmission for cars, trucks motorcycles and the like, where the working fluid acts in large part as a hydraulic transfer fluid.

The working fluids of the invention are particularly valuable as automatic transmission fluids in, e.g., automatic transmissions under high stress conditions which require the high performance obtainable with these fluids. Obvious examples include transmissions in racing vehicles such as those used in drag racing, heavy duty pickup trucks, off road vehicles, delivery trucks, monster trucks etc.

Typically the polyol ester composition comprises a mixture of compounds of formula I

wherein each R is independently an alkyl carbonyl of from 5 to 10 carbon atoms, each R₁ is independently selected and is either a group R or a substituent of formula II:

wherein R is as above.

Typically the polyol ester composition comprises a mixture of compounds of formula Ia and IIa;

wherein each R is independently an alkyl carbonyl of from 5 to 10 carbon atoms, i.e., pentanoyl, hexanoyl, heptanoly, octanoyl, nonanoyl, decanoyl and branched isomers thereof.

The ratios of the various alkyl carbonyl groups can vary widely, but in most embodiments, there is typically no more than 75% of any single 5, 6, 7, 8, 9 or 10 carbon alkanoyl. There also is no requirement that the composition contain each of a 5, 6, 7, 8, 9 and 10 carbon alkanoyl, although such a composition may be present. In some embodiments the various alkanoyl groups that are present are found in roughly equal amounts, in other embodiments, certain alkanoyl groups may predominate. Often, however, there are at least 3, 4 or 5 alkanoyl groups of differing numbers of carbon atoms present in the compounds of formulas Ia and IIa.

While R can be selected from branched and unbranched alkyl carbonyl, in many embodiments the majority of R groups are unbranched, i.e., linear. For example, in some embodiments 60 to 100% or 70 to 100% of the R groups are linear, and in particular embodiments 80 to 100%, 90% to 100%, or 95 to 100% of the R groups are linear.

Often, the majority of the esters of the working fluid are compounds of formula Ia and IIa. For example, in some embodiments 60 to 100% or 70 to 100% of the esters of the working fluid are compounds of formula Ia and IIa and in particular embodiments 80 to 100%, 90 to 100%, or 95 to 100% of the esters of the working fluid are compounds of formula Ia and IIa

Compounds of formula Ia are typically the major portion of the esters, for example, ratios of compound of formula Ia to those of IIa often range from 1.1:1 to 15:1, for example, from 1.5:1 to 12:1, from 2:1 to 10:1. For example, the ratio of compounds of formula Ia to IIa is typically at least 2:1, 3:1, 4:1 5:1 or 6:1, and can be as high as 15:1, 12:1, 10:1 or 8:1.

For example, in many embodiments the working fluid comprises:

-   i) from 80 to 99% by weight of a mixture of compounds of formula Ia     and IIa wherein:     -   a) from 0 to 50 mol %, 5 to 45 mol % 5 to 40 mol %, or 10 to 40         mol % of all R groups are C₅ alkanoyl;     -   b) from 0 to 50 mol %, 5 to 45 mol % 5 to 40 mol %, or 10 to 40         mol % of all R groups are C₆ alkanoyl;     -   c) from 0 to 50 mol %, 5 to 45 mol % 5 to 40 mol %, or 10 to 40         mol % of all R groups are C₇ alkanoyl;     -   d) from 0 to 50 mol %, 5 to 45 mol % 5 to 40 mol %, or 10 to 40         mol % of all R groups are C₉ alkanoyl;     -   e) from 0 to 50 mol %, 5 to 45 mol % 5 to 40 mol %, or 10 to 40         mol % of all R groups are C₉ alkanoyl; and/or     -   f) from 0 to 50 mol %, 5 to 45 mol %, 5 to 40 mol %, or 10 to 40         mol % of all R groups are C₁₀ alkanoyl; provided that at least 3         of a), b), c), d), e) and f) are present in at least 5 mol %,         and     -   ii) from 1 to 20% additives selected from dispersants; anti-wear         agents; anti-oxidants; corrosion inhibitors; friction modifiers;         seal swell agents; anti-foamants viscosity modifiers,         detergents, pour point depressants, such as those typically         found in automatic transmission fluids.

In a particular embodiment, the polyol ester composition (i) of the working fluid comprises compounds of formula Ia and IIa wherein:

-   -   from 25 to 40 mol % of all R groups are C₅ alkanoyl;     -   from 15 to 30 mol %, of all R groups are C₇ alkanoyl;     -   from 15 to 30 mol %, of all R groups are C₉ alkanoyl; and     -   from 0 to 30 mol % of all R groups are selected from C₆, C₈ and         C₁₀ alkanoyl.

The polyester mixtures of the invention, and processes for preparing them, are known in the art. They are however complex and are best described as a product by process defined in part by their physical properties. As the polyol esters are the major part of the present working fluids, the fluid is also best described, in part, by physical properties such as viscosities at 40° C., 100° C., viscosity index and pour point. For example, the working fluid of the invention can be defined in part as having a kinematic viscosity at 40 ° C. of from 4 to 64 cSt, a kinematic viscosity at 100° C. of from 2 to 28 cSt, a viscosity index of greater than greater than 100 and a pour point of −25° C. or lower.

In some embodiments the viscosity index is at least 110, in others at least 120, or at least 130, and in particular embodiments the viscosity index is 140 or higher.

Likewise, the kinematic viscosity at 40° C. in some embodiments is from 5 to 48 cSt, in others from 10 to 40 cSt, e.g., from 12 to 32 cSt.

In some embodiments the kinematic viscosity at 100° C. is from 2 to 10 cSt, in other embodiments from 2 to 8 or 2 to 6 cSt, e.g. from 4 to 8.

Various embodiments also include those where the pour point is −30° C. or lower, −40° C. or lower and in some embodiments −50° C. or lower.

The polyol esters of the invention are present in the working fluid in amounts of from 80 to 99 wt %. In some embodiments the polyol esters are present at 85 wt % or more, often 90% or more, e.g., 95% or more. While the upper limit of polyol esters is 99 wt %, upper limits of 98%, 97% or 95% are common. Likewise, the additives may be present in amounts of 1 to 15%, 1 to 10%, 1 to 5%, 2 to 15%, 3 to 15% or 5 to 15%. The individual amounts of each additive are those common in the art and can range from 0.01% to 7% by weight.

The working fluids of the invention are particularly valuable in, e.g., automatic transmissions under high stress conditions which require the high performance obtainable with these fluids. Obvious examples include transmissions in racing vehicles such as those used in drag racing, heavy duty pickup trucks, off road vehicles, delivery trucks, monster trucks etc.

Some of the particular embodiments of the invention are to transmissions that comprise a fluid coupling, for example, a torque converter, and a working fluid which functions at least in part as a hydraulic transfer fluid, e.g., an automatic transmission fluid, comprising or consisting essentially of the working fluid described above, in particular, said transmissions designed to operate in or more of the following conditions:

-   A) high stall speeds as encountered in drag racing vehicles -   B) high rpm, e.g., 4,000 rpm, 5,000 rpm or 6,000 rpm or higher -   C) high horse power, e.g., 300, 350, 400, 450 HP or higher -   D) high torque, e.g., 400, 450, 500, 600, 700, 800 ft/lb or higher -   E) high loads -   F) significant low gear use -   G) more than 30, 40 or 50% stop and go conditions, or -   H) high temperatures or wide temperature ranges.

EXAMPLES

In the following examples, a working fluid according to the invention is compared to commercial fluids useful in high stress automatic racing transmissions which operate under conditions of high horsepower, high rpm, high torque and high temperature. Comparative Example A is a purchased, fully formulated, commercial product, VALVOLINE DEXRON VI AUTOMATIC TRANSMISSION FLUID, which uses petroleum hydrotreated paraffin distillates mineral oil as the base oil. Comparative Example B is a purchased, fully formulated, commercial product, AMSOIL synthetic PC series compressor oil, which uses synthetic lubricants such as poly-alpha olefins and polyol esters as the base oil. The compressor oil is not formulated as an automatic transmission fluid per se, but has been used in dragsters and monster truck transmissions due to its ability to function under high heat conditions.

The working fluid of the invention contains, by weight, 95% of a base oil consisting essentially of esters of pentanoyl, hexanoyl, heptanoyl, octanoyl and nonanoyl esters of formula Ia and IIa, in a weight ratio of about 10:1 to about 15:1 of Ia to IIa wherein, from 30 to 40 mol % of all R groups are pentanoyl, from 15 to 25 mol %, of all R groups are C₇ heptanoyl, from 15 to 25 mol %, of all R groups are C₉ nonanoyl, and the remainder of R groups being selected from hexanoyl and octanoyl; and as additives 3.4% aromatic amine anti-oxidants; 1% triazole and azeleic acid corrosion inhibitors; 0.2% anti-wear additives comprising a mixture of ester, chloromethyl phosphonic acid and a primary amine; and 0.2% dispersant/viscosity improver/pour point depressant comprising a polymerized/grafted mixture of methacrylates, ester, mercaptan, heptanes, and N-vinyl-2-pyrrolidone.

Viscosity of the test fluids:

KV@ 40° C. KV@ 100° C. VI Invention 27 cSt 5.2 cSt 125 Comp A 30 cSt 6.0 cSt 150 Comp B 34 cSt 6.3 cSt 140 Transmission performance

The working fluid of the invention described above was added to the automatic transmission comprising a torque convertor, of a turbo charged, non-nitro driving racing vehicle and exhibited excellent performance relative to other fluids tested. The converter tightened with the inventive fluid, which allowed the turbos to spool up faster with more boost; better coupling at launch with no foaming issues was found; and the inventive fluid resulted in a reduction in transmission fluid operating temperatures of approximately 30° F.

Cameron Plint Friction

The friction coefficient of each fluid was measured, using standard Cameron Plint Tribology methods, at variety of temperatures, the conditions and data are in the tables below:

Test Conditions:

Ramp Hold Stage load N, kg Temp ° C. Time min. Time min. Frequency Hz 1 0, 0  25-35 10 5 0 2 50, 5.1 35-50 10 5 5 3 100, 10.2  50-165 60 0 5

Friction Data:

Friction coefficient (—) at 102° C. 132° C. 162° C. Invention 0.095 0.105 0.115 COMP A 0.083 0.083 0.061 COMP B 0.090 0.092 0.088

Wear

The fluids were tested for anti-wear activity according to ASTM D-4172, four ball wear test. The fluids were also tested for anti-wear activity using a Cameron Plint TE-77 instrument by gradually heating a sample of the fluid on a steel plate to 150° C. at which temperature the sample is held for 5 min, while a ball with a 100 Newton load is rubbed against the plate at a frequency of 30 Hz throughout the entire heating protocol. In each test the wear is indicated by measuring scar formation.

4-Ball Scar Cameron Plint Scar Invention 0.766 mm 0.966 mm COMP A 0.579 mm 0.582 mm COMP B 0.469 mm 0.392 mm

The working fluid of the invention surprisingly outperforms the comparison fluids when used as an automatic transmission fluid in the transmission tested even though the comparison fluids seem to have better friction and wear properties. 

1. A mechanical device comprising a fluid coupling and a hydraulic transfer fluid, said hydraulic transfer fluid being a working fluid comprising: i) from 80 to 99% by weight of a polyol ester composition comprising C₅₋₁₀ alkylcarboxlate esters of pentaerythritol, di-pentaerythritol, tri-pentaerythritol and/or higher pentaerythritol oligomers; and ii) from 1 to 20% by weight of additives selected from dispersants; anti-wear agents; anti-oxidants; corrosion inhibitors; friction modifiers; seal swell agents; anti-foamants viscosity modifiers, detergents, pour point depressants; wherein the working fluid has a kinematic viscosity at 40° C. of from 4 to 64 cSt, a kinematic viscosity at 100° C. of from 2 to 28 cSt, and a viscosity index of greater than greater than
 100. 2. The mechanical device according to claim 1 wherein 70 to 100% by weight of the esters of the polyol ester composition (i) are compounds of formula Ia and IIa

wherein each R is independently selected from the group consisting of pentanoyl, hexanoyl, heptanoly, octanoyl, nonanoyl, decanoyl and branched isomers thereof.
 3. The mechanical device according to claim 2 wherein 80 to 100 mol % of the R groups in formula (Ia) and (IIa) are linear alkanoyl.
 4. The mechanical device according to claim 1 wherein the working fluid has a kinematic viscosity at 40 ° C. of from 10 to 40 cSt, a kinematic viscosity at 100 ° C. of from 2 to 10 cSt and a pour point of −25° C. or lower
 5. The mechanical device according to claim 3 wherein 90 to 100% of the esters of the polyol ester composition (i) are compounds of formula Ia and IIa in a weight ratio of compounds of formula Ia to IIa of from 2:1 to 15:1.
 6. The mechanical device according to claim 5 wherein the weight ratio of compounds of formula Ia to IIa in the polyol ester composition (i) is from 6:1 to 15:1.
 7. The mechanical device according to claim 6 wherein the polyol ester composition (i) of the working fluid comprises compounds of formula Ia and IIa wherein: a) from 5 to 45 mol % of all R groups are C₅ alkanoyl; b) from 5 to 45 mol %, of all R groups are C₈ alkanoyl; c) from 5 to 45 mol %, of all R groups are C₇ alkanoyl; d) from 5 to 45 mol %, of all R groups are C₈ alkanoyl; e) from 5 to 45 mol %, of all R groups are C₉ alkanoyl; and/or f) from 5 to 45 mol %, of all R groups are C₁₀ alkanoyl; provided that at least 3 of a), b), c), d), e) and f) are present in at least 5 mol %.
 8. The mechanical device according to claim 7 wherein the polyol ester composition (i) of the working fluid comprises compounds of formula Ia and IIa wherein: from 25 to 40 mol % of all R groups are C₅ alkanoyl; from 15 to 30 mol %, of all R groups are C₇ alkanoyl; from 15 to 30 mol %, of all R groups are C₉ alkanoyl; and from 0 to 30 mol % of all R groups are selected from C₈, C₈ and C₁₀ alkanoyl
 9. The mechanical device according to claim 8 wherein the polyol ester composition (i) of the working fluid comprises compounds of formula Ia and IIa wherein: from 30 to 40 mol % of all R groups are pentanoyl, from 15 to 25 mol % of all R groups are C₇ heptanoyl, from 15 to 25 mol %, of all R groups are C₉ nonanoyl, and the remainder of R groups are selected from hexanoyl and octanoyl.
 10. The mechanical device according to claim 1 wherein the fluid coupling is torque converter.
 11. The mechanical device according to claim 10 which is an automatic transmission.
 12. The automatic transmission according to claim 11 which is an automatic transmission in a racing vehicle, pickup truck, off road vehicle, delivery truck, ATV, or monster truck.
 13. The automatic transmission according to claim 12 which is an automatic transmission in a car used in drag racing or monster truck.
 14. An improved method for operating an automatic transmission comprising a fluid coupling and a hydraulic transfer fluid, the improvement being selecting as the hydraulic transfer fluid a working fluid comprising: i) from 80 to 99% by weight of a polyol ester composition comprising C₅₋₁₀ alkylcarboxlate esters of pentaerythritol, di-pentaerythritol, tri-pentaerythritol and/or higher pentaerythritol oligomers; and ii) from 1 to 20% by weight of additives selected from dispersants; anti-wear agents; anti-oxidants; corrosion inhibitors; friction modifiers; seal swell agents; anti-foamants viscosity modifiers, detergents, pour point depressants; wherein the working fluid has a kinematic viscosity at 40° C. of from 4 to 64 cSt, a kinematic viscosity at 100° C. of from 2 to 28 cSt, and a viscosity index of greater than greater than
 100. 15. The improved method according to claim 14 wherein 80 to 100% by weight of the esters of the polyol ester composition (i) are compounds of formula Ia and IIa

wherein each R is independently selected from the group consisting of pentanoyl, hexanoyl, heptanoly, octanoyl, nonanoyl, decanoyl and branched isomers thereof.
 16. The improved method according to claim 15 wherein 80 to 100 mol % of the R groups in formula (Ia) and (IIa) are linear alkanoyl.
 17. The improved method according to claim 16 wherein 90 to 100% of the esters of the polyol ester composition (i) are compounds of formula Ia and IIa in a weight ratio of compounds of formula Ia to IIa of from 6:1 to 15:1.
 18. The improved method according to claim 17 wherein the polyol ester composition (i) of the working fluid comprises compounds of formula Ia and IIa wherein: a) from 5 to 45 mol% of all R groups are C₅ alkanoyl; b) from 5 to 45 mol %, of all R groups are C₆ alkanoyl; c) from 5 to 45 mol %, of all R groups are C₇ alkanoyl; d) from 5 to 45 mol %, of all R groups are C₈ alkanoyl; e) from 5 to 45 mol %, of all R groups are C₉ alkanoyl; and/or f) from 5 to 45 mol %, of all R groups are C₁₀ alkanoyl; provided that at least 3 of a), b), c), d), e) and f) are present in at least 5 mol %.
 19. The improved method according to claim 18 wherein the polyol ester composition (i) of the working fluid comprises compounds of formula Ia and IIa wherein: from 25 to 40 mol % of all R groups are C₅ alkanoyl; from 15 to 30 mol %, of all R groups are C₇ alkanoyl; from 15 to 30 mol %, of all R groups are C₉ alkanoyl; and from 0 to 30 mol % of all R groups are selected from C₆, C₈ and C₁₀ alkanoyl
 20. The improved method according to claim 19 wherein the polyol ester composition (i) of the working fluid comprises compounds of formula Ia and IIa wherein: from 30 to 40 mol % of all R groups are pentanoyl, from 15 to 25 mol % of all R groups are C₇ heptanoyl, from 15 to 25 mol %, of all R groups are C₉ nonanoyl, and the remainder of R groups are selected from hexanoyl and octanoyl. 